A laser is a device that has the ability to produce coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium, which has typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator that is defined by highly reflective surfaces that form a closed round-trip path for light, and the active medium is contained within the optical cavity.
Laser diodes are the most ubiquitous types of lasers available today. They are compact, rugged and relatively inexpensive, and are therefore potentially useful in many commercial applications. Output powers available from single-mode laser diodes such as single-stripe diodes are as high as several hundred milliwatts (mW). These low power outputs limit their applications. On the other hand, output powers available from semiconductor laser devices with large active areas, such as broad-area laser diodes, phased array diodes, and tapered amplifiers, are on the order of several watts. Because of their relatively large active areas as compared to single-stripe laser diodes, broad-area laser diodes, phased array diodes, tapered amplifiers are hereafter referred to as large-area diodes.
Although large-area diodes are significantly more powerful than single-stripe diodes, in many applications powers much higher than the output powers of large-area diodes are desirable. The generation of blue light through non-linear optical processes is one of such cases. Due to limitations of material technology, laser diodes directly generating visible lights in the range of blue are difficult to make. Non-linear optical processes are practical ways for generating blue lights. For instance, infrared output of a large-area diode laser can be used in a second harmonic generation (SHG) process to produce blue light. Blue light can also be produced by frequency-summing two less energetic lights. The efficiencies of those non-linear optical processes depend strongly on the power of the input light. For example, the conversion efficiency of the second harmonic generation process is proportional to the square of the power of the light to be converted. It is therefore advantageous to be able to concentrate the output of a laser diode to produce a light power that is much higher than the output power of the diode.
One way to concentrate the output power of a laser is to use a passive resonator, which is basically an optical cavity defined by reflective mirrors, to store or concentrate the laser light. Depending on the design of the passive resonator and the optical coupling between the laser and the passive resonator, the power in the passive resonator can be orders of magnitude higher than the direct output power of the diode.
Concentrating the output of a laser diode in a passive resonator cavity can be a difficult process, however. In order to efficiently introduce the output of a semiconductor laser diode into the passive resonator, the output of the laser diode has to be mode-matched to a resonant mode of the passive cavity. For efficient mode-matching, the laser diode has to operate at a single frequency and in a well-defined spatial mode. One example of a design that provides concentrated power in a passive resonator locked to a single-strip diode is described and illustrated in applicant's related U.S. patent application Ser. No. 08/370,508, filed Jan. 9, 1995. Unfortunately, the beams of higher power broad-area and phased array laser diodes are characterized by poor spatial and spectral qualities. Specifically, the output beam of a broad-area or phased array diode is not diffraction limited in the direction parallel to the junction of the diode, and the beam typically includes many different frequencies, which correspond to the different longitudinal modes of the diode resonator.
Although the poor spatial and spectral qualities of the broad-area diode lasers and phased arrays diodes make them unsuitable for many applications, spatial beam cleanup techniques are known for producing near-diffraction-limited beams. In general, the resonant cavity of a broad-area diode laser or laser diode array is extended beyond the output facets of the diode, and spatial beam cleanup is accomplished by placing a spatial filter in the portion of the extended laser cavity outside the semiconductor element. In addition, the circulating field in the extended cavity is incident on the output facet of the gain element at an angle to the normal that is typically between two (2) and five (5) degrees. Both linear and ring architectures for these extended cavities have been reported. U.S. Pat. No. 4,905,252 to Goldberg et al. illustrates an example of such an extended cavity. Other examples can be found in the following references: C. J. Chang-Hasnain, J. Berger, D. R. Scifres, W. Streifer, J. R. Whinnery and A. Dienes, "High Power and High Efficiency In A Narrow Single-Lobed Beam From a Diode Laser Array In An External Cavity, " Applied Phys. Lett. 50 1465 (1987); L. Goldberg, J. F. Weller, and M. K. Chung, "Diffraction-Limited Broad Stripe Laser Emission In An External Resonator," Digest of the Conference on Lasers and Electro-Optics (Optical Society of America, Washington, D.C., 1989), Paper FL6.
Although the beam cleanup techniques described above can be used to obtain single-frequency near-diffraction-limited beams from extended cavity broad-area and phased array diode lasers, difficulties remain in mode-matching the output of the diode laser to the passive resonator. In order to achieve maximum power buildup in a passive optical cavity, the input radiation has to be both spatially and spectrally mode-matched to the resonance of the cavity. Spatial mode-matching generally involves the use of optical elements to adjust the shape and size of the laser output beam to match the fundamental transverse mode of the passive resonator. Spectral mode-matching requires that the frequency of the laser output be matched to a resonant frequency of the passive resonator. If this frequency-matching condition is not satisfied, the laser output that is transmitted into the passive resonator will not be able to build up inside the passive resonator. The frequency mismatch between the laser diode and the passive resonator can be caused by imperfections in a normal operating environment, such as mechanical vibrations, temperature variations, and the like, which are typically called "technical noises." Those technical noises cause changes in the resonant frequencies of the passive resonator and the frequency of the laser diode, thereby affecting the mode-matching between the laser diode and the resonator.
Complex electronic stabilization techniques like Drever-Pound locking are typically used to keep the frequency of a laser diode locked to a resonance mode of a passive resonator. Those techniques are generally called active locking due to their use of active components. Those electronic locking techniques are not satisfactory because they are incapable of maintaining lock for an extended period of time when the laser system is subject to technical noises found in normal operational environments. Their optoelectronic complexities and high costs also make them unsuitable for commercial applications.